Greenhouse Gas Mitigation Options for California Agriculture.

Recently, agricultural food production has been more linked to emissions of greenhouse gases (GHG) because of its contribution to destruction of the ozone and global change. In California alone, agriculture is responsible for 6% of the total GHG emissions, with 12.5% emitted as CO2, 37.5% as CH4, and 50% as N2O. The main sources of these gases are fossil fuel consumption, biomass burning, soil degradation, livestock, manure, anaerobic soils (mostly in rice production), and inorganic and organic fertilizer. While many of the above practices are unavoidable, over the past several years, growers and researchers have teamed up to develop economically feasible practices to mitigate GHG emissions within California agriculture. Some of these practices include reduced or zero tillage, conversion from annual to perennial grasses, winter cover cropping, increased hay in crop rotations, higher residue (both above- and below-ground) yielding crops, manure applications and organic cropping systems, improved fertilizer management, and the application of biochar to soils. In California, several of these practices are currently used although many others have yet to be seriously considered by growers until convincing results are produced, both in terms of significant reduction in GHG emissions as well as no significant decrease in yield. One of the main challenges before effective mitigation practices can be developed is obtaining accurate estimates of our current GHG emissions due to the high temporal and spatial variation, especially of N2O, across crop types, soil types, and climates. In our studies, we measured background N2O emissions from two rarely studied perennial cropping systems prevalent in California agriculture: vineyards and almond orchards; compared conventional versus integrated management in tomato crops; and tested the effect of biochar amendments on annual and seasonal N2O emissions within vegetable crops and walnut orchards. Current vineyard N2O emissions without and with a leguminous cover crop ranged from 0.2 to 2.0 kg N2O-N ha-1 yr-1, respectively, while the almond orchard averaged only 0.4 kg N2O-N ha-1 yr-1. The conventional tomato cropping system yielded 2.1 kg N2O-N ha-1 yr-1 while the integrated system, which used subsurface drip irrigation compared to furrow irrigation produced less than half that, with only 0.9 kg N2O-N ha-1 yr-1. Preliminary results from the walnut biochar study show slight increases in N2O emissions compared to the untreated control, however, reduced emissions were found when the same biochar was applied to lettuce crops. These results indicate the need for further research in this area, coupled with more accurate field measurements in order to foster adoption of these practices among farmers as well as policy makers.